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CHICAGO — Surprising research suggests that childhood cancer is most common in the Northeast, results that even caught experts off-guard. But some specialists say it could just reflect differences in reporting.

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The large government study is the first to find notable regional differences in pediatric cancer. Experts say it also provides important information to bolster smaller studies, confirming that cancer is rare in children, but also more common in older kids, especially among white boys.

The study from the Centers for Disease Control and Prevention is based on data representing 90 percent of the U.S. population. It found that cancer affects about 166 out of every million children, a number that shows just how rare childhood cancers are.

The highest rate was in the Northeast with 179 cases per million children, while the lowest was among children in the South with 159 cases per million. Some experts suggested that could mean there is better access to care in the urban centers of the Northeast, leading to more diagnoses.

The rates for the Midwest and West were nearly identical, at 166 cases per million and 165 per million, respectively.

The cancer incidence in boys was 174 cases per million, compared with 157 cases per million in girls. In white children, the rate was 173 per million, versus 164 per million in Hispanics and 118 per million in blacks. Teenagers had higher rates than younger kids.

A total of 36,446 cases were identified in the study, which analyzed 2001-03 data from state and federal registries. The research appears in the June edition of Pediatrics, released Monday.

"It's very powerful that this study includes so much of the U.S. population so it gives us a good picture of where we are with the incidence of these childhood cancers,'' said Elizabeth Ward, the American Cancer Society's surveillance director.

Experts said the regional differences, though small, are intriguing, but that reasons for them are uncertain.

Better care, more diagnoses?Dr. Rafael Ducos, a children's cancer physician at Ochsner Medical Center in New Orleans, said the South's low rates were perplexing and might simply reflect under-reporting there and over-reporting in other regions.

"I'm at a loss to explain it,'' he said.

Environmental factors might play a role, including exposure to radiation, said lead author Dr. Jun Li of the CDC. Radiation has been linked with the most common types of childhood cancer _ leukemia, lymphoma and brain cancers.

Radiation sources include X-rays, nuclear plant emissions and natural sources such as radon gas. But Li said research is needed to determine if these sources vary enough by region to affect childhood cancer rates.

But also, there could be better access to cancer centers in the Northeast, which would result in more diagnoses, she said. That could also explain why other research has shown that children's death rates from cancer are also lowest in the Northeast.

While noteworthy, the differences in rates among regions shouldn't cause alarm among parents, said Dr. Adam Levy, a cancer specialist at the Children's Hospital at Montefiore Medical Center in New York.

"As a parent raising a family in the Northeast, this does not at all increase my concern for my family or for my neighbors,'' Levy said, adding, "First and foremost, these are still very rare diseases in children.''

Regional differences in rates for some specific cancers have been found in adults, but these are likely due to personal habits and lifestyle factors, Ward said. For example, lung cancer rates are high in the South because smoking is generally more popular there, she said.

But it generally takes years of exposure to lifestyle factors such as smoking before cancer develops, she said, so this wouldn't explain children's rates.